Abstract

Sol-gel auto-combustion technique was used to synthesize spinel ferrite nanoparticles of Mn0.2Co0.8Fe2O4 (MCF). Using the modified Stöber method, these magnetic nanoparticles were encapsulated with silica to form the core/shell Mn0.2Co0.8Fe2O4/SiO2 (MCFS). The phase composition, morphology, particle size, and saturation magnetization of the encapsulated nanoparticles were studied using X-ray diffraction (XRD), high resolution-transition electron microscopy (HR-TEM), and vibrating sample magnetometer (VSM). HR-TEM images indicated that particle size of the nanoparticles ranged from 15 to 40 nm, and VSM measurements showed that Ms of uncoated and coated samples were 65.668 emu/g and 61.950 emu/g and the Hc values were 2,151.9 Oe and 2,422.0 Oe, respectively. The effects of metal concentration, solution pH, contact time, and adsorbent dose of the synthesized nanoparticles on lead (Pb2+) ions removal from an aqueous solution were investigated. Based on Langmuir isotherm model, the results for peak adsorption capacity of the adsorbent under optimal conditions was 250.5 mg/g and 247 mg/g for MCF and MCFS, respectively. We concluded that Pb2+ adsorption occurred via a chemisorption mechanism based on the analysis of adsorption kinetics. The adsorbents displayed consistent adsorption efficiencies following three cycles of regeneration, indicating that these magnetic nanoparticles are promising candidates for wastewater purification.

INTRODUCTION

Water pollution from dyes and heavy metals has become a crucial problem due to the toxicity of these materials on human health (Badruddoza et al. 2013). Main concerns include evidence that most of them are carcinogenic, highly toxic, and mutagenic, even at relatively low concentrations (Ahmed et al. 2013; Gómez-Pastora et al. 2014). Additionally, the development of several disorders and diseases have been linked to the presence of heavy metals in living organisms as a result of bioaccumulation (Gotoh et al. 2004; Badruddoza et al. 2013). Pb(II) is especially significant as it is one of the most widely distributed metal ions in the environment and is widely used in many industrial materials such as pigments, fertilizers, and battery manufacturing (Yang et al. 2010; Duan et al. 2015). Pb2+ ions are considered a heavy metal hazard in environmental and clinical health due to low biodegradation. Acute lead exposure can cause headache, renal dysfunction, and fatigue, while chronic exposure can cause birth defects, allergies, weight loss, and kidney damage (Ozdes et al. 2009; Duan et al. 2015).

Several approaches have been investigated and adopted for heavy metal removal including ion exchange, filtration, reverse osmosis, and adsorption (Zhou et al. 2009). Among these methods, adsorption has some advantages such as simplicity, safety, low-cost, high efficiency, and biological regeneration (Monier et al. 2010). For example, agricultural wastes such as peanut and rice husk and mango kernel bio-composite have been introduced as efficient economical adsorbents to remove Sr(II), Cr(VI), and U(VI) ions, respectively, from aqueous solution (Akram et al. 2017; Kausar et al. 2017; Kausar et al. 2018). Recently, F.K. Mahar et al. reported a safe, easy, and quick adsorption method for lead ions via porous carbon nanofiber-based polyacrylonitrile polymer with high adsorption capability (7.1 mg/g) at neutral pH within 1 min (Mahar et al. 2019). Hybrid ion exchangers (Saruchi & Kumar 2019), composite hydrogels (Jiang et al. 2019), and sorbents modified with surface functional groups (Koksharov et al. 2019) were found to be efficient adsorbents for lead ions removal with excellent adsorption capacity reaching 182.7 mg/g for lead ions (Saruchi & Kumar 2019). Additionally, the use of magnetic nanoparticles has received a great deal of attention in removing environmental pollutants (Gómez-Pastora et al. 2014). This is a result of the improved chemical and physical properties of magnetic nanoparticles, such as chemical stability, higher surface area, ease of functionalization, and the easy separation in complex multiphase systems using an external magnetic field (Badruddoza et al. 2013). Cobalt ferrite has many unique properties that make it a viable option for the reduction of environmental contamination, including high coercivity (Hc), specific saturation magnetization (Ms), high Curie temperature, mechanical hardness, and chemical stability (Salunkhe et al. 2014). Furthermore, it has been reported that the addition of Mn2+ to CoFe2O4 causes Co2+ ion migration, resulting in structural changes making it a suitable material for the treatment of industrial wastes (Ahalya et al. 2014). Silica shell coatings provide some protection to the cobalt ferrite core from rapid biodegradation and spontaneous aggregation (Pon-On et al. 2011; Karimi et al. 2013). It also allows a wide range of temperature operations, biocompatibility, and dielectric properties with a nontoxic surface (Vivekanandhan et al. 2013).

In the present study, cobalt manganese ferrite (MCF) magnetic nanoparticles were prepared and encapsulated with a silica shell (MCFS). Then, we explored the potential use of these magnetic nanoparticles for the efficient removal of Pb2+ from aqueous solution at different operating conditions such as contact time, pH, and adsorbent dose. Comprehensive isotherm and kinetic studies of Pb2+ adsorption on the surfaces of MCF and MCFS nanoparticles were conducted. The experimental data were applied for the linear forms of different isotherm models such as Langmuir, Freundlich, and Dubinin–Radushkevich to determine the adsorption capacity. The adsorption mechanism was determined by applying the kinetics models namely the pseudo-first-order model and pseudo-second-order model.

MATERIALS AND METHODS

To synthesize the nanoparticles, the following chemicals were commercially purchased and used. (Fe(NO3)3·9H2O, ferric nitrate, Mw = 404.00 g/mol, SDFCL, Mumbai, India), (Co(NO3)2·6H2O, cobalt (II) nitrate extra pure, Mw = 291.03 g/mol, Alpha Chemika, India), (Mn(NO3)2 4H2O, manganese(II) nitrate, Mw = 251.01 g/mol, Sigma Aldrich, Missouri, USA), (C6H8O7·H2O, citric acid monohydrate gritty, Puriss, Mw = 210.14 g/mol, SDFCL, Mumbai, India), ammonia solution (30%), and C8H20O4Si, tetraethyl orthosilicate (TEOS), Mw = 208.33 g/mol, Sigma Aldrich, Missouri, USA).

Synthesis of MCF and MCFS samples

As reported by the authors in a previous work (Awad et al. 2015), MCF nanoparticles were successfully synthesized via the sol-gel auto-combustion method. Briefly, solutions of ferric, cobalt, and manganese nitrates were prepared by dissolving the metal nitrate powders in distilled water, then these solutions were mixed in a certain molar ratio under constant stirring to form homogenous solutions under heating rate of 5 °C/min up to 80 °C for 1 h. Citric acid and ammonia solution were added to adjust the pH, and the reaction temperature was adjusted to form the viscous gel and initiate the auto-combustion. The yield ash followed by a post synthesis drying at 100 °C for 24 h, then fired at 500 °C for 2 h. MCFS nanoparticles were synthesized by the same method, then encapsulated with silica to form the core/shell structure of Mn0.2Co0.8Fe2O4/SiO using the modified Stöber method.

Batch adsorption experiments

A given quantity of the absorbent at a working Pb2+ solution pH was used in the batch adsorption experiment. For initial Pb2+ concentrations of 20 and 100 mg/L, the influence of contact time (5, 10, 15, 20, 30, 60, and 120 min) was investigated at room temperature (25 °C) and agitation at 200 rpm in a rotary shaker. The pH of the solution was adjusted by 0.1 N HNO3 and 0.1 N NaOH. The solution's Pb2+ concentration was deduced using Atomic Absorption Spectrometer (Varian Spectra AAS 220) fitted with an arc background corrector of deuterium, according to APHA (2005). An absorption calibration curve was performed for each measurement series. The adsorbed quantity of Pb2+ per unit mass of the adsorbent was quantified mathematically via the mass balance equation (Azzam et al. 2016): 
formula
(1)
where qe is the adsorption capacity at equilibrium (mg/g), V is volume of the liquid, W is the used adsorbent mass (g), Ce is the equilibrium liquid-phase concentrations of solute in aqueous solution (mg/L), and C0 is the initial liquid-phase concentration of solute in aqueous solution (mg/L).

Isotherm studies

Different initial metal ions concentration, ranging from 50 mg/L to 250 mg/L at the adsorbent optimal dose and effective pH were used in adsorption isotherm studies. All experiments were duplicated to confirm reproducibility. To evaluate the equilibrium adsorption of compounds from the solution, we used three different models; the Dubinin-Kaganer-Radushkevich (DKR) isotherm model, Freundlich isotherm model, and Langmuir isotherm model. In the Langmuir adsorption isotherm, adsorption on the surface occurs in a monolayer consisting of a finite number of identical sites and can be described in the non-linear expression as (Langmuir 1918): 
formula
(2)
where qe (mg/g) is the corresponding adsorption capacity; qm (mg/g) and KL (L/mg) are constants which are related to adsorption capacity and energy or net enthalpy of adsorption, respectively. This isotherm model involves saturated adsorption, meaning monolayer coverage on the surface, hence additional adsorption cannot take place.
Adsorption isotherms were also evaluated by the Freundlich model which assumes that the adsorption process occurs on a heterogeneous surface and the adsorbent surface contain different binding energy sites and can be described in the non-linear form by (Freundlich 1906): 
formula
(3)
where kF is the adsorption capacity (mg/g), n is the level of control Ce has on adsorption.
The DKR model is utilized to outline the adsorption process on heterogeneous surfaces as well as homogenous surfaces. The model is described by the formula (Hutson & Yang 1997): 
formula
(4)
 
formula
(5)
where ɛ is the Polanyi potential, R (J/mol K) is the gas constant; and T (K) is the absolute temperature. The constant KDR (mol2/KJ2) is the activity coefficient that gives the mean free energy of sorption per molecule of the sorbate, E (kJ/mol) when it is transferred from infinity in the solution to the surface of the solid and can be computed using the following equation: 
formula
(6)

Kinetic modeling

Different kinetic models (pseudo-first order and pseudo-second order models) were employed to the experimental data in order to study the adsorption mechanism. The pseudo-first order model describes the adsorption in solid/liquid systems for a given range of the solid sorption potential as shown in the equation (Ho 2004): 
formula
(7)
where k1 represents the first-order adsorption rate constant (min−1), and qt is the adsorbed metal amount at time t, (mg/g). The second-order kinetic model can be described by the following equation in which K2 is the pseudo-second order rate constant of adsorption (g/mg. min) (Ho 2006): 
formula
(8)

Desorption analysis

In order to determine lead desorption and the reusability of the adsorbents, lead-loaded absorbents were used for lead desorption and their reusability was tested (Azzam et al. 2016). Distilled water (DW) and NaOH-NaCl mixture solutions were used as eluents for lead desorption from the lead-loaded absorbents. For sorbent reusability test, a known volume of 20 mg/L Pb solution was adsorbed by a certain amount of lead-loaded adsorbent for 1 h. Lead-loaded absorbents used to desorb lead ions by the introduction of a known volume of DW and 0.1 N HNO3. The absorbents were then washed in DW until neutrality was achieved. Then, the adsorbents were dried and reconditioned to test the reusability of the prepared adsorbents. The experiment was repeated four times over successive adsorption–desorption cycles. The equation that calculates the efficiency of lead recovery R (%) from the solid phase is as follows: 
formula
(9)
where Cdes is the quantity of Pb2+ discharged into the solution and Cads is the quantity of Pb2+ adsorbed by the adsorbents (mg/L).

RESULTS AND DISCUSSION

Particle size and phase composition of the prepared nanoparticles

The phase composition and purity of the prepared nanoparticles were confirmed by the X-ray diffraction (XRD) analysis. Figure 1(a) shows the XRD pattern of MCF and MCFS magnetic nanoparticles. The main crystal planes of (111), (220), (311), (222), (400), (422), (511), (440), (620), and (533) indicate a cubic spinel structure of MSF nanoparticles. Also, no secondary phases were detected, confirming the purity of the formed nanoparticles. High resolution-transition electron microscopy (HR-TEM) images indicated that the dimension of the nanoparticles range from 15 to 40 nm as shown in Figure 1(b) and 1(c), which correlated with previous reports in other studies (Awad et al. 2015). These HR-TEM images confirmed the nano-sized particles of MCF and MCFS which appeared to be agglomerated due to the magnetic nature of these particles. The magnetic properties of samples were recorded at room temperature using a vibrating sample magnetometer (VSM). Magnetic parameters such as saturation magnetization (Ms) and coercivity (Hc) were obtained from the hysteresis loops of the prepared nanoparticles. Ms of uncoated and coated samples were 65.668 emu/g and 61.950 emu/g and the Hc values were 2,151.9 Oe and 2,422.0 Oe, respectively. These results reflect the ferromagnetic behavior of the MCF and MCFS nanoparticles. This property allows an external magnet to efficiently extract the nanoparticles and adsorbed lead.

Figure 1

XRD patterns of Mn0.2Co0.8Fe2O4 and Mn0.2Co0.8Fe2O4/SiO2 nano-particles (a) and HR-TEM images indicating the average particle size of the prepared nano-particles (b) and (c).

Figure 1

XRD patterns of Mn0.2Co0.8Fe2O4 and Mn0.2Co0.8Fe2O4/SiO2 nano-particles (a) and HR-TEM images indicating the average particle size of the prepared nano-particles (b) and (c).

Effects of initial concentration and contact time

The results revealed that the removal percentage on both adsorbents increases with contact time. During the first 10 mins, Pb2+ was rapidly absorbed, decreasing slowly until saturation. At the beginning of the adsorption process, there were many free sites on the adsorbent. As the experiment progressed, lead was bound to free sites, reducing total free sites. This makes additional binding increasingly difficult.

Equilibrium was achieved after 60 min for Pb2+ at working pH = 5.7 for both adsorbents. Also, adsorption removal % decreased as the initial metal concentration increased. As shown in Figure 2, high lead removal percentages were obtained using MCF sample, but the silica shell coating decreased the metal adsorption ability of MCFS to a small extent compared to the uncoated MCF sample.

Figure 2

Effect of contact time and initial concentration on the adsorption of Pb2+ at different concentrations (absorbent dose: 0.5 g/L, temperature: C, pH: 5.7, and C0: 20 and 100 mg/L).

Figure 2

Effect of contact time and initial concentration on the adsorption of Pb2+ at different concentrations (absorbent dose: 0.5 g/L, temperature: C, pH: 5.7, and C0: 20 and 100 mg/L).

Effect of initial pH

Figure 3 illustrates the removal percentage (%) curve as a function of pH, indicating that the removal percentage increased as the pH increased for both adsorbents for an adsorbent dose and lead concentration of 0.5 g/L and 20 mg/L, respectively. Lead removal increased from 48.7% to 98.4% at pH 2 and 6.5, respectively, using the MCF sample. At low pH value, there was competition between the hydronium ion (H3O+) and lead ion. This is due to the positive charging of the adsorbent surface, resulting in a significant decrease of lead ion adsorption. Increasing the pH led to a reduction in hydronium ion concentration, resulting in increased adsorption of other ions in solution. This experiment indicates that the pH range is limited to pH ≤ 6.5 due to the precipitation of Pb2+ in the form of Pb(OH)2 at higher pH (from 6.5 to 8).

Figure 3

Effect of pH on the adsorption of Pb2+ by MCF and MCFS (absorbent dose: 0.5 g/L, temperature: 25° C and C0: 20 mg/L).

Figure 3

Effect of pH on the adsorption of Pb2+ by MCF and MCFS (absorbent dose: 0.5 g/L, temperature: 25° C and C0: 20 mg/L).

Effect of adsorbent dose

The effect of adsorbent does was evaluated. Pb2+ removal at room temperature was carried out for 0.05–1.0 g/L adsorbent dose range, at pH of 6.5 and Pb2+ concentration of 20 mg/L, as shown in Figure 4. Increased lead uptake was seen with increasing adsorbent dose, from 0.1 until 0.5 g/L, and equilibrium was attained at 0.5 g/L. No additional effect was seen above 0.5 g/L.

Figure 4

Effect of adsorbent dose on the adsorption of Pb2+ by MCF and MCFS (pH 6.5, temperature: 25° C and C0: 20 mg/L).

Figure 4

Effect of adsorbent dose on the adsorption of Pb2+ by MCF and MCFS (pH 6.5, temperature: 25° C and C0: 20 mg/L).

ISOTHERM STUDIES

The equilibrium isotherm study is an essential step to develop an appropriate isotherm model. Thus, the current study used three different models to conduct the analysis. Figure 5 shows the non-linear fitting of Langmuir, Freund-lich, and DKR adsorption isotherm models of Pb ions onto MCF and MCFS adsorbents (Figure 5(a) and 5(b), respectively). The correlation coefficient (R2 = 0.99) for the Langmuir model suggests a meaningful relationship and favorable adsorption. Lead maximum adsorption capacities were 250.5 and 247 mg/g for MCF and MCFS samples, respectively, as indicated in Table 1. These values are near the experimental adsorbed amounts and correspond closely to the adsorption isotherm plateau, which indicates that Langmuir model can be used to describe the adsorption of lead ions by both adsorbents. This suggests that a strong case can be made for monolayer adsorption from the experimental data. On the other hand, from the analysis of the experimental data based on the Freundlich and DKR models, the 1/n value obtained from the Freundlich model is <1, indicating a good adsorption. However, the metal ions investigated in this study showed greater correlation coefficient with the Langmuir isotherm (R2 > 0.98) when compared to the correlation with the Freundlich isotherm (R2 ∼ 0.95 and 0.92) and DKR (R2 ∼ 0.90). Although the DKR model shows a high correlation coefficient (R2 ∼ 0.97) for the MCFS sample, the values of theoretical saturation capacity are all lower than the experimental amounts corresponding to the adsorption isotherm plateau, indicating that this model is unacceptable for the current adsorption. The mechanism of the adsorption reaction can be derived from the magnitude of E. For E values between 8 and 16 kJ/mol, chemical-ion exchange is the primary process for adsorption. For E values above 16 kJ/mol, chemical adsorption becomes the dominant form of adsorption, with ion exchange playing a negligible role. For E values below 8 kJ/mol, adsorption is mostly influenced by physical forces. In the present study, E values were 15.9 and 15.1 for MCF and MCFS samples, respectively, confirming that the adsorption occurred via a chemical ion-exchange mechanism.

Table 1

Freundlich, Langmuir, and DKR isothermal adsorption equation parameters for the adsorption of Pb2+ by MCF and MCFS nanoparticles at room temperature (absorbent dose: 0.5 g/L, pH value: 6.5, metals concentration: 50–250 mg/L, contact time: 5–60 min, agitation speed: 200 rpm)

MCFMCFS
Pb2+Pb2+
Freundlich isotherm parameters 
 l/n 0.217 0.23 
KF (mg/g) 91.8 79.7 
R2 0.95 0.92 
Langmuir isotherm parameters 
 qmax (mg/g) 250.5 247 
b (L/mg) 0.18 0.11 
R2 0.99 0.98 
DKR isotherm parameters 
qmax (mol/g) 218 205 
 KDR 2.4 × 10−6 3.9 × 10−6 
 E (KJ/mol) 15.9 15.14 
 R2 0.90 0.97 
MCFMCFS
Pb2+Pb2+
Freundlich isotherm parameters 
 l/n 0.217 0.23 
KF (mg/g) 91.8 79.7 
R2 0.95 0.92 
Langmuir isotherm parameters 
 qmax (mg/g) 250.5 247 
b (L/mg) 0.18 0.11 
R2 0.99 0.98 
DKR isotherm parameters 
qmax (mol/g) 218 205 
 KDR 2.4 × 10−6 3.9 × 10−6 
 E (KJ/mol) 15.9 15.14 
 R2 0.90 0.97 
Figure 5

Non-linear fitting of Langmuir, Freundlich, and DKR adsorption isotherm models of Pb onto MCF (a) and MCFS (b) adsorbents (absorbent dose: 0.5 g/L; pH value: 6.5; initial concentration: 50–250 mg/L; contact time: 5–60 min; agitation speed: 200 rpm).

Figure 5

Non-linear fitting of Langmuir, Freundlich, and DKR adsorption isotherm models of Pb onto MCF (a) and MCFS (b) adsorbents (absorbent dose: 0.5 g/L; pH value: 6.5; initial concentration: 50–250 mg/L; contact time: 5–60 min; agitation speed: 200 rpm).

Heavy metals adsorption capacity of CoFe2O4 itself and in combination with other molecules has been investigated in several studies. In comparison to the results reported in literature, the Pb2+ adsorption capacities of MCF and MCFS (250.5 and 247 mg/g, respectively) nanoparticles introduced in this study were higher than those found in a similar study of CoFe2O4 by Culita et al. (2015). Culita et al. demonstrated the adsorption capacities of Pb2+ by CoFe2O4-CTAB 0.05M and CoFe2O4-CTAB 0.01M to be 32.11 mg/g and 26.41 mg/g, respectively, according to the Langmuir model. Also, the adsorption capacities of the prepared adsorbents are comparable to (CoFe2O4)-reduced graphene oxide (rGO) as investigated by Zhang et al. (2014). Furthermore, the adsorption capacities of the MCF and MCFS samples outperformed other magnetic nanoparticles that have been investigated in numerous studies (Hua et al. 2012). To name a few, iron oxide returned a result of 42.4 mg/g for Pb2+ removal, hydrated ferric oxide reported 211.4 mg/g Pb2+ removal, manganese oxide resulted in 99 mg/g Pb2+ removal (Hua et al. 2012), and magnetic cellulose acetate showed 44 mg/g Pb2+ removal (Shalaby et al. 2016). Further, MCF and MCFS nanoparticles showed higher adsorption capacities compared to hybrid ion-exchanger adsorbent that was used in a recent study (82.3 and 182.7 mg/g for lead ions removal) (Saruchi & Kumar 2019). Also, our investigations indicated that the Pb2+ removal increased from 48.7% to 98.4% when the pH increased from 2 to 6.5, respectively. Similar results were obtained by Zhang et al. (2014) for Pb2+ removal by CoFe2O4-rGO, as the pH > 4.0 resulted in an increase of the removal efficiency of Pb2+ by 85%. Culita et al. (2015) reported that the amount of Pb2+ adsorbed by CoFe2O4 increased when the pH increased from 2 to 5. Also, Viltužnik et al. (2013) reported that increasing the initial pH value of solution up to 7 resulted in a gradual increase in sorption capacity.

Adsorption kinetics

Different kinetic models were applied to evaluate the mechanism of the adsorption process. Pseudo-first-order and pseudo-second-order models were used to interpret the experimental data. Fittings of kinetic models are shown in Figure 6(a) and 6(b) and the obtained parameters are shown in Table 2. The parameters obtained from the first-order rate Equation (7) (Ho 2004) are given in Table 2. It was found that the correlation coefficient values are low, and the calculated values of adsorption capacity (qe) using this model were less than the experimental values. Consequently, the adsorption cannot be classified as first-order kinetic. On the other hand, lead adsorption on the MCF and MCFS samples was found to follow the second-order kinetic Equation (8) (HO 2006), where the experimental and calculated values of qe agreed with each other with higher correlation coefficients compared to the first-order rate. Based on the adsorption kinetics data, it is concluded that the adsorption of lead ions by the MCF and MCFS magnetic nanoparticles follows the second-order kinetic equation as confirmed from the agreement of the experimental and calculated data.

Table 2

Kinetic parameters for the adsorption of Pb2+ by MCF and MCFS nanoparticles at room temperature (absorbent dose: 0.5 g/L, pH value: 6.5, metals concentration: 50–250 mg/L, contact time: 5–60 min, agitations speed: 200 rpm)

MCFMCFS
Pb2+Pb2+
Pseudo-first-order 
qe (mg/g) (calculated) 30 35 
qe (mg/g) (experiment) 38.4 38 
K1 (min−10.49 0.45 
R2 0.77 0.80 
Pseudo-second-order 
qe (mg/g) (calculated) 37.9 37.6 
qe (mg/g) (experiment) 38.4 38.00 
 K2 (g/mgmin) 0.036 0.031 
R2 0.88 0.82 
MCFMCFS
Pb2+Pb2+
Pseudo-first-order 
qe (mg/g) (calculated) 30 35 
qe (mg/g) (experiment) 38.4 38 
K1 (min−10.49 0.45 
R2 0.77 0.80 
Pseudo-second-order 
qe (mg/g) (calculated) 37.9 37.6 
qe (mg/g) (experiment) 38.4 38.00 
 K2 (g/mgmin) 0.036 0.031 
R2 0.88 0.82 
Figure 6

Shows the adsorption kinetic models: pseudo-first-order and pseudo-second-order models for Pb2+ onto MCF (a) and MCFS (b) nanoparticles (absorbent dose: 0.5 g/L; pH value: 6.5; initial concentration: 20 mg/L; contact time: 5–60 min; agitation speed: 200 rpm).

Figure 6

Shows the adsorption kinetic models: pseudo-first-order and pseudo-second-order models for Pb2+ onto MCF (a) and MCFS (b) nanoparticles (absorbent dose: 0.5 g/L; pH value: 6.5; initial concentration: 20 mg/L; contact time: 5–60 min; agitation speed: 200 rpm).

DESORPTION AND REUSE STUDIES

In the industrial practice of metal ions removal from wastewater, the reuse of adsorbents for repeated cycles is crucial for the reliability of the adsorbents. Experiments were conducted for the regeneration of both adsorbents (MCF and MCFS) using two solutions; mixtures of NaOH-NaCl and DW. Regenerated adsorbents were used for adsorption in successive cycles. It was noted that lead recovery from MCF is easier than MCFS, and the recovery efficiencies of Pb2+ loaded-MCF and MCFS adsorbents by a NaCl and NaOH mixture were better than those obtained using DW, even after three adsorption–desorption cycles. The recovery efficiencies of Pb2+ reached 70% using a NaCl-NaOH mixture and 65% using DW for three cycles in MCF as shown in Figure 7. For MCFS, after three cycles the lead recovery percentage reached 68% and 64% by NaCl-NaOH mixture and DW, respectively. Both adsorbents kept their adsorption efficiency for three cycles of the sorption–desorption process. Based on the high adsorption capacity and reproducibility of the prepared materials, these magnetic nanoparticles could be promising adsorbents for the removal of Pb2+ ions.

Figure 7

Shows the desorption and reuse studies of MCF and MCFS adsorbents.

Figure 7

Shows the desorption and reuse studies of MCF and MCFS adsorbents.

CONCLUSIONS

This study demonstrates that MCF and MCFS nanoparticles are effective adsorbents for the removal of Pb2+ from aqueous solutions. The sol-gel auto-combustion technique was used to prepare the magnetic nanoparticles that were then coated with silica. XRD analysis showed the formation of spinel structures of the nanoparticles. Particle size observed by HR-TEM ranged from 15 to 40 nm. The magnetic studies of the prepared nanoparticles showed that the saturation magnetization of uncoated and coated samples were 65.668 emu/g and 61.950 emu/g, and the coercivity values were 2,151.9 Oe and 2,422.0 Oe, respectively. Lead adsorption results aligned well with the Langmuir and Freundlich models, and the adsorption kinetics confirmed that the adsorption best match with the second-order model indicating a chemisorption reaction. Also, both MCF and MCFS nanoparticles adsorbents outperformed many of the reported studies in the Pb2+ adsorption capacities (MCF = 250.5 and MCFS = 247 mg/g). Furthermore, these adsorbents maintained adsorption efficiency after three cycles of regeneration. The current study concluded that MCF and MCFS magnetic nanoparticles are promising candidates for lead ions removal from wastewater.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the National Research Center at Egypt for supporting and funding this study. We also want to acknowledge Mr Mathew Fiedler (University of Texas at Arlington) for his technical support and proofreading of this manuscript.

REFERENCES

REFERENCES
Ahalya
K.
,
Suriyanarayanan
N.
&
Ranjithkumar
V.
2014
Effect of cobalt substitution on structural and magnetic properties and chromium adsorption of manganese ferrite nano particles
.
Journal of Magnetism and Magnetic Materials
372
,
208
213
.
Ahmed
M. A.
,
Ali
S. M.
,
El-Dek
S. I.
&
Galal
A.
2013
Magnetite–hematite nanoparticles prepared by green methods for heavy metal ions removal from water
.
Materials Science and Engineering: B
178
(
10
),
744
751
.
Akram
M.
,
Bhatti
H. N.
,
Iqbal
M.
,
Noreen
S.
&
Sadaf
S.
2017
Biocomposite efficiency for Cr(VI) adsorption: kinetic, equilibrium and thermodynamics studies
.
Journal of Environmental Chemical Engineering
5
(
1
),
400
411
.
APHA
2005
Standard Methods for the Examination of Water and Wastewater
, 21st edn.
American Public Health Association
,
Washington, DC, USA
.
Awad
K. R.
,
Wahsh
M. M. S.
,
Othman
A. G. M.
,
Girgis
E.
,
Mabrouk
M. R.
&
Morsy
F. A.
2015
Effect of Mn2+ doping and SiO2 coating on magneto-optical properties of CoFe2O4 nano-particles
.
Smart Materials and Structures
24
(
11
),
115002
.
Azzam
A. M.
,
El-Wakeel
S. T.
,
Mostafa
B. B.
&
El-Shahat
M. F.
2016
Removal of Pb, Cd, Cu and Ni from aqueous solution using nano scale zero valent iron particles
.
Journal of Environmental Chemical Engineering
4
(
2
),
2196
2206
.
Badruddoza
A. Z. M.
,
Shawon
Z. B. Z.
,
Tay
W. J. D.
,
Hidajat
K.
&
Uddin
M. S.
2013
Fe3O4/cyclodextrin polymer nanocomposites for selective heavy metals removal from industrial wastewater
.
Carbohydrate Polymers
91
(
1
),
322
332
.
Culita
D. C.
,
Simonescu
C. M.
,
Dragne
M.
,
Stanica
N.
,
Munteanu
C.
,
Preda
S.
&
Oprea
O.
2015
Effect of surfactant concentration on textural, morphological and magnetic properties of CoFe2O4 nanoparticles and evaluation of their adsorptive capacity for Pb(II) ions
.
Ceramics International
41
(
10
),
13553
13560
.
Duan
S.
,
Tang
R.
,
Xue
Z.
,
Zhang
X.
,
Zhao
Y.
,
Zhang
W.
,
Zhang
J.
,
Wang
B.
,
Zeng
S.
&
Sun
D.
2015
Effective removal of Pb(II) using magnetic Co0.6Fe2.4O4 micro-particles as the adsorbent: synthesis and study on the kinetic and thermodynamic behaviors for its adsorption
.
Colloids and Surfaces A: Physicochemical and Engineering Aspects
469
,
211
223
.
Freundlich
H.
1906
Adsorption in solution
.
Physical Chemical Society
40
,
1361
1368
.
Gotoh
T.
,
Matsushima
K.
&
Kikuchi
K.-I.
2004
Adsorption of Cu and Mn on covalently cross-linked alginate gel beads
.
Chemosphere
55
(
1
),
57
64
.
Ho
Y.
2006
Review of second-order models for adsorption systems
.
Journal of Hazardous Materials
136
(
3
),
681
689
.
Hua
M.
,
Zhang
S.
,
Pan
B.
,
Zhang
W.
,
Lv
L.
&
Zhang
Q.
2012
Heavy metal removal from water/wastewater by nanosized metal oxides: a review
.
Journal of Hazardous Materials
211–212
,
317
331
.
Jiang
C.
,
Wang
X.
,
Wang
G.
,
Hao
C.
,
Li
X.
&
Li
T.
2019
Adsorption performance of a polysaccharide composite hydrogel based on crosslinked glucan/chitosan for heavy metal ions
.
Composites Part B: Engineering
169
,
45
54
.
Karimi
Z.
,
Karimi
L.
&
Shokrollahi
H.
2013
Nano-magnetic particles used in biomedicine: core and coating materials
.
Materials Science and Engineering: C
33
(
5
),
2465
2475
.
Kausar
A.
,
MacKinnon
G.
,
Alharthi
A.
,
Hargreaves
J.
,
Bhatti
H. N.
&
Iqbal
M.
2018
A green approach for the removal of Sr(II) from aqueous media: kinetics, isotherms and thermodynamic studies
.
Journal of Molecular Liquids
257
,
164
172
.
Koksharov
S. A.
,
Aleeva
S. V.
&
Lepilova
O. V.
2019
Description of adsorption interactions of lead ions with functional groups of pectin-containing substances
.
Journal of Molecular Liquids
283
,
606
616
.
Langmuir
I.
1918
The adsorption of gases on plane surfaces of glass, mica and platinum
.
Journal of the American Chemical Society
40
(
9
),
1361
1403
.
Mahar
F. K.
,
He
L.
,
Wei
K.
,
Mehdi
M.
,
Zhu
M.
,
Gu
J.
,
Zhang
K.
,
Khatri
Z.
&
Kim
I.
2019
Rapid adsorption of lead ions using porous carbon nanofibers
.
Chemosphere
225
,
360
367
.
Monier
M.
,
Ayad
D. M.
&
Sarhan
A. A.
2010
Adsorption of Cu(II), Hg(II), and Ni(II) ions by modified natural wool chelating fibers
.
Journal of Hazardous Materials
176
(
1–3
),
348
355
.
Ozdes
D.
,
Gundogdu
A.
,
Kemer
B.
,
Duran
C.
,
Senturk
H. B.
&
Soylak
M.
2009
Removal of Pb(II) ions from aqueous solution by a waste mud from copper mine industry: equilibrium, kinetic and thermodynamic study
.
Journal of Hazardous Materials
166
(
2–3
),
1480
1487
.
Pon-On
W.
,
Charoenphandhu
N.
,
Tang
I.-M.
,
Jongwattanapisan
P.
,
Krishnamra
N.
&
Hoonsawat
R.
2011
Encapsulation of magnetic CoFe2O4 in SiO2 nanocomposites using hydroxyapatite as templates: a drug delivery system
.
Materials Chemistry and Physics
131
(
1–2
),
485
494
.
Salunkhe
A. B.
,
Khot
V. M.
,
Phadatare
M. R.
,
Thorat
N. D.
,
Joshi
R. S.
,
Yadav
H. M.
&
Pawar
S. H.
2014
Low temperature combustion synthesis and magnetostructural properties of Co–Mn nanoferrites
.
Journal of Magnetism and Magnetic Materials
352
,
91
98
.
Viltužnik
B.
,
Košak
A.
,
Zub
Y. L.
&
Lobnik
A.
2013
Removal of Pb(II) ions from aqueous systems using thiol-functionalized cobalt-ferrite magnetic nanoparticles
.
Journal of Sol-Gel Science and Technology
68
(
3
),
365
373
.
Vivekanandhan
S.
,
Venkateswarlu
M.
,
Carnahan
D.
,
Misra
M.
,
Mohanty
A. K.
&
Satyanarayana
N.
2013
Sol–gel mediated surface modification of nanocrystalline NiFe2O4 spinel powders with amorphous SiO2
.
Ceramics International
39
(
4
),
4105
4111
.
Yang
S.
,
Zhao
D.
,
Zhang
H.
,
Lu
S.
,
Chen
L.
&
Yu
X.
2010
Impact of environmental conditions on the sorption behavior of Pb(II) in Na-bentonite suspensions
.
Journal of Hazardous Materials
183
(
1–3
),
632
640
.
Zhang
Y.
,
Yan
L.
,
Xu
W.
,
Guo
X.
,
Cui
L.
,
Gao
L.
,
Wei
Q.
&
Du
B.
2014
Adsorption of Pb(II) and Hg(II) from aqueous solution using magnetic CoFe2O4-reduced graphene oxide
.
Journal of Molecular Liquids
191
,
177
182
.